Can't send email from outside

I use ISPconfig v3.0.16 for a long time.
Today I found I can't send email from outside network.
I can't connect the ispconfig server use
telnet mail.mymail.com 25
from outside network
but I can connect it in the local network.
I have set the DMZ to the ispconfig server.
and check the Centos5.3 have no firewall.
anyone can give me hits
thank you very much !!!

But we use business line, all ports is not block by ISP.
And all incoming and outgoing email have no problem.
Because some staff need send email from home though
the ispconfig email server. So we found that problem.

Do I need post some information for yours help ?
Thank you very much !!!